A base plate for the backrest of a chair may preferably have a curved support surface fitting the back of an occupant for a chair. For example, JP2000-270962A and JP2001-128788A disclose a base plate for the backrest of a chair having a flexible curved surface following the back of the occupant.
The base plate for the backrest in the Japanese publications is likely to bend depending on the back of the occupant, but it is not sufficient to provide flexibility in the intermediate part mainly supporting the back of the occupant. It is necessary to improve sitting comfort.